[dm-devel] dm-writecache: change config parameters using messages

[Mikulas Patocka]

On Thu, 7 Nov 2019, Maged Mokhtar wrote:

> 
> 
> On 07/11/2019 21:09, Mike Snitzer wrote:
> > On Thu, Nov 07 2019 at  1:55pm -0500,
> > Maged Mokhtar <mmokhtar at petasan.org> wrote:
> > 
> > > 
> > > 
> > > On 06/11/2019 17:08, Mike Snitzer wrote:
> > > > On Tue, Nov 05 2019 at  4:19pm -0500,
> > > > Maged Mokhtar <mmokhtar at petasan.org> wrote:
> > > > 
> > > > > Gentle ping please.
> > > > > 
> > > > > It could add flexibility in changing cache parameters after device
> > > > > creation.
> > > > 
> > > > I'm inclined to _not_ take this type of change.
> > > > 
> > > > Why isn't changing the config parameters via table reload viable for
> > > > you?
> > > > 
> > > 
> > > 
> > > Hi Mike,
> > > 
> > > Thank you for your response. The main issue is to enable setting
> > > some config parameters while the device is mounted and running and
> > > avoid calling target ctr() by sending parameter changes via
> > > messages. A similar setup was used in dm-cache.
> > > 
> > > The reason is that tuning the write cache may require run time
> > > changes. If un-tuned we can observes periods of spikes and/or step
> > > like disk utilization on the slow device during writeback using
> > > iostat, and these spikes correspond to periods of increased client
> > > io latency. Utilization can be tuned by varying the number of active
> > > writeback jobs + the gap between high and low marks to achieve a
> > > smooth high utilization.  Such tuning is difficult to do when
> > > creating the cache device as it depends on the hardware and io
> > > workload. We are hoping to use some userspace monitoring and tuning
> > > tool to periodically adjust these values while the device is
> > > running.
> > 
> > I think you're missing that any actively used DM device can be
> > suspended, table reloaded, resumed.  So the tuning at runtime is still
> > possible, it just requires more steps.
> > 
> > And I'm saying that unless/until there is a better reason other than
> > "dm-cache allowed tuning via messages" I'm not interested in having
> > multiple methods for tuning dm-writecache.
> > 
> > Mike
> > 
> just for my understanding, does not table reload call _ctr() and
> re-initializes things like threads/read metadada ..?

Yes it does. But you don't have to unmount the filesystem when you reload 
the table.

Mikulas